Process for making fibres from poly(p-phenylene terephthalamide)

ABSTRACT

The invention relates to an improvement of the known process for spinning poly(p-phenylene terephthalamide) via an air gap spinning process. It comprises mixing poly(p-phenylene terephthalamide) with concentrated sulphuric acid by a freezing process, heating the solid mixture, and passing the resulting solution to the spinning orifices under pressure. It involves the use of a mixing kneader to convert the solid mixture into a spinning solution.

BACKGROUND OF THE INVENTION

The invention relates to a process for making fibres from a polyamideconsisting wholly or for the most part of poly(p-phenyleneterephthalamide) in which a spinning mass is prepared by coolingconcentrated sulphuric acid of at least 98 wt. % content to below itscoagulation point, subsequently combining the thus cooled sulphuric acidwith the polyamide and mixing them to form a solid mixture containing,calculated on the weight of the mixture, at least 15% of the polyamidehaving an inherent viscosity of at least 2.5, and then heating theresulting solid mixture and passing it, under pressure, to spinningorifices, and spinning it by means of an air gap spinning process.

Such a process is known from Netherlands patent application 7904495 laidopen to public inspection. It was found that with the aid of thisso-called freezing process proper mixing of sulphuric acid andpoly(p-phenylene terephthalamide) could be achieved. However, theresulting sandy solution lacks sufficient homogeneity for immediateprocessing after being melted. For this reason the sandy solution isheated in an extruder prior to being spun. Not all drawbacks areobviated by this procedure, however. Since not every polymer granulewill absorb the same quantity of sulphuric acid, the sulphuricacid:polymer ratio on a microscopic scale differs from the setmacroscopic scale ratio. This will give inhomogeneities during melting.Furthermore, it was found that favourable results in actual practice arehighly dependent on precise settings for the pressure build-up and thevariation in temperature in the extruder. In consequence, there is aconsiderable risk of failure of the extruder process. It was also foundto be advisable in actual practice to compress the sandy solution in thecold state, which severely curtails the options to increase or changethe process capacity.

It has also been proposed to prevent inhomogeneities in thepoly(p-phenylene terephthalamide) spinning solution by means of animproved process of dissolving poly(p-phenylene terephthalamide) insulphuric acid. A two-step mixing process to this end in whichpoly(p-phenylene terephthalamide) and sulphuric acid are successivelymixed in a small twin-screw mixer operated at high speed and a largetwin-screw mixer operated at low speed is known from Research Disclosure232 004. Such a process has nothing to do with eliminating the drawbacksto the known freezing process, nor does it have the advantages providedby said process.

From WO 92/07120 it is known to modify a poly(p-phenyleneterephthalamide) spinning solution by the addition thereto of solutioncontaining additives. A static mixer unit is employed to this end. Thisprocess has nothing to do with improving the aforementioned freezingprocess either.

DESCRIPTION OF THE INVENTION

The invention has for its object to eliminate the drawbacks to the knownfreezing process. To this end the invention consists in that in aprocess of the known type mentioned in the opening paragraph the heatingand pressurising of the solid mixture is carried out in a device inwhich the polymer solution passes through at least two successive zones,i.e., a melting zone and a pressure build-up zone, with the polymersolution being kneaded as well as mixed in at least the melting zone.

Since in the melting zone the polymer solution is transformed from agranular structure (containing many voids) into a liquid, thedegasification of the polymer solution by means of a vacuum preferablytakes place in this zone.

The conventional extruders combine a number of functions, i.e.,kneading, mixing, melting, degasification, and pressure build-up. It wasfound that not only the process capacity and controllability, but alsothe homogeneity of the spinning solution, the quality of the yarn, andthe loop and cord strengths can be enhanced by dividing these functionsbetween at least two separate zones, a melting zone and a pressurebuild-up zone.

Further, it was found that the process according to the presentinvention allows the production of spinning solutions having a higherconcentration of poly(p-phenylene terephthalamide) than was possiblewith a process using an extruder. Spinning of solutions having a higherconcentration of poly(p-phenylene terephthalamide) results in filamentshaving improved physical properties (higher modulus and strength).Consequently, by enabling the use of higher concentrations ofpoly(p-phenylene terephthalamide), the process according to the presentinvention provides an additional improvement (to those alreadymentioned) of the physical properties.

A device pre-eminently suited to be used as a melting zone is the"mixing kneader." Mixing kneaders are known from, int. al., CH 673 617,CH 661 450, EP 422 454, and EP 451 747. Preferably, a continuouslyoperating mixing kneader such as described in EP 451 747 is employed.Such mixing kneaders (available from LIST) have the advantage ofcomparatively good plug flow characteristics, i.e., a low dwell timedistribution. In the process according to the invention the dwell timein the mixing kneader can vary from 15 minutes to several hours, with adwell time of more than 30 minutes being preferred. The mixing kneaderusually operates at a filling level ranging from 25 to 70%.

Mixing kneaders such as those preferably used in the process accordingto the invention comprise a hollow, rotating shaft equipped with anumber of kneading paddles or kneading blades. The shaft is arranged ina cylindrical housing optionally having a number of kneading hooks onthe inside. The paddles and hooks are arranged relatively to one anotherin such a way as will give no, or the lowest possible number, of deadpockets and prevent encrustation of the shaft or the cylindricalhousing. In addition, a heating medium, e.g., water, flows through theshaft and, optionally, the paddles or blades mounted on it. Thecylindrical housing can also be heated.

Needless to say, the transport of the polymer solution through themixing kneader has to proceed at a certain rate. Such can be attained,e.g., by placing several paddles or blades along a single helical line.In that case, the movement of the polymer solution will be effected bythe arrangement of the paddles relatively to one another. Such amovement can also be achieved by mounting the paddles or blades on therotating shaft each along their own helical line. The paddles or blades(or sections of these, such as scraping devices) will then be positionedat an angle to the longitudinal axis of the rotating shaft and will eachindividually bring about a movement of the polymer solution.

Most of the offset lines are "positive." This means that movement in thedirection of transport is promoted. However, it is also possible to haveone or more "negative" offset lines (promoting the movement in thedirection opposite to the direction of transport). These "negative"offset lines can be used, e.g., to adjust the dwell time distributionand the filling level distribution as desired.

When the mixing with kneading takes place under a vacuum, there is verythorough degasification of the spinning solution. In order not todisturb the vacuum, the mixing kneader can be fed alternatingly from twobunkers in vacuum, which are connected to the filler inlet of the mixingkneader by turns. Alternatively, the mixing kneader can be fed using avacuum lock which periodically lets through a certain quantity ofpolymer solution.

On leaving the melting zone, the polymer solution enters the pressurebuild-up zone. While a wide range of pumps is suitable for use in thepressure build-up zone, the pump preferably used is a so-called "boosterpump," since a pump of this type operates at a low suction pressure andin addition provides a high pressure build-up.

Since the vast majority of pumps suitable for use in said pressurebuild-up zone require a certain pre-pressure (say, 2 or 6 bar), it isnecessary as a rule, and especially with degasification under a vacuum,to increase the pressure in the polymer solution before it reaches thepump. For the pressure build-up use can be made of, e.g., a "lobe pump."

However, it has been found that the pressure in the polymer solution caneasily be raised by providing a discharge screw at the end of the mixingkneader's rotating shaft. By using such a discharge screw the pressurein the polymer solution can be increased sufficiently, thus obviatingthe need for an additional pump together with driving mechanism.

The pressure build-up can also be favourably affected by cooling thefinal section of the mixing kneader.

To increase to some extent the filling level of the section of themixing kneader in which the discharge screw is present, a baffle can beplaced just upstream of the discharge screw.

In addition, the end of the discharge screw can be fitted with a viscousseal, so that the polymer solution will be backed up at the end of thedischarge screw. On the one hand this guarantees that the section of themixing kneader containing the discharge screw remains sufficientlyfilled, on the other it prevents the glands between the rotating shaftand the cylindrical housing from being overloaded or over-contaminated.

The process according to the present invention will be illustrated byway of the embodiment below. For details of a preferred continuous plugflow mixing which are not discussed reference may be had to Europeanpatent application no. 451 747 mentioned hereinbefore.

The mixing kneader is made up of a cylindrical vessel equipped withstationary transport paddles, kneading paddles, kneading hooks, aninlet, a discharge screw, and a baffle. Via the inlet the sandy polymersolution is alternatingly fed to the mixing kneader at room temperature.The temperature can be measured at different sites using temperaturemeasuring points.

In the melting zone, which is equipped with kneading paddles, kneadinghooks, and an opening for creating a vacuum, the polymer solution ismelted at about 80°-95° C. and kneaded.

Next, the polymer solution is discharged via the discharge screw. Thepolymer solution is fed to a booster pump, where the pressure isincreased sufficiently to pass the spinning solution to the spinningpumps.

The spinning solution is then passed via a central filter to thespinning orifices, where it is subjected to a conventional air gapspinning process such as described in, e.g., U.S. Pat. No. 3,414,645 andU.S. Pat. No. 4,016,236.

The process according to the present invention was found to have variousadvantages. For instance, a very coarse solution or a solutioncontaining a large quantity of inhomogeneities and even undissolvedmaterial can also be processed without any problem. Besides, as comparedwith the known process there is a major improvement in the spinningsolution's homogeneity on a micro scale. It was further found to beeasier to increase the capacity of the process according to the presentinvention, since virtually the only thing that needs to be taken intoconsideration is the required melting heat of the solution.

Alternatively, the mixing kneader can be fed a polymer solution having ahigher polymer concentration than the final spinning solution. In thatcase, sulphuric acid or oleum can be added to the mixing kneader toadjust the spinning solution concentration at a much later stage than ispossible when use is made of an extruder. This enhances the process'scontrollability and prevents the process having to be shut down onaccount of too great variations in the polymer concentration.

It has further been found that the yarn spun by the process according tothe present invention is of improved quality. Another significant resultis the improvement found in greige cord strength and loop strength.

It is noted that W094/06530 describes a process for making a veryspecific cellulose solution by means of a thin-film evaporator which, inturn, is very specifically described. W094/06530 neither relates tosandy aramid solutions nor addresses the problems involved in using anextruder to process said sandy solutions.

It is further noted that U.S. Pat. No. 3,873,072 describes an apparatusfor kneading a wax-like substance into a molten polymer. Within saidapparatus the wax and the polymer are kneaded such that the wax isuniformly finely dispersed in the polymer as a separate and distinctphase. The wax particle size and distribution should fall within aspecified range so as to ensure optimum protection against build-up ofstatic electricity. U.S. Pat. No. 3,873,072 fails to mention or suggestthat when it is combined with a separate pressure build-up device, theapparatus for mixing and kneading can advantageously be used to solvethe problems related to the use of an extruder in processing sandyaramid solutions made with the freezing process.

The invention will be further illustrated with reference to thefollowing unlimitative example.

EXAMPLE

The mixing kneader employed was a LIST DTB-60 with four compartments anda mixing shaft. Between the first and second compartments was placed ablind baffle to reduce the overall operating volume of the mixingkneader. Between the second and third compartments and the third andfourth compartments, respectively, was placed a baffle having across-section of 11% and 42%, respectively, of the mixing kneadercross-section. In the fourth compartment the mixing shaft was fittedwith a discharge screw.

The wall of the second compartment (the melting zone) had a temperatureof 88° C., the wall of the third compartment (the degasification zone)and of the fourth compartment had a temperature of 85° C. The mixingshaft was kept at a temperature of 90° C. and rotated at a speed of 40revolutions per minute. The mixing kneader was kept under a vacuum of 30mbar, the filling level was set at 50% ±5.

Via a vacuum lock having a feeding capacity of 40 kg/h a solid polymersolution consisting of 80.5 wt. % of sulphuric acid and 19.5 wt. % ofpoly(p-phenylene terephthalamide) was introduced into the secondcompartment of the mixing kneader. The dwell time of the polymersolution was about 90 minutes.

The molten spinning solution was fed to the spinning orifices via abooster pump (Transmark™) and a transport pump (Slack & Parr™) with athroughput of 40 kg/h and spun into filament yarns by means of aconventional air gap spinning process.

The filaments obtained by the mixing kneader and extruder processes havethe following properties (determined in accordance with ASTM standardD885M-95):

    ______________________________________                    Mixing kneader                             Extruder    ______________________________________    Yarn linear density                   (dtex) 1715       1726    Number of filaments                   (--)   1000       1000    Breaking force (N)    356        345    Elongation at break                   (%)    3.55       3.50    Loop breaking strength                   (N)    314        290    Loop breaking  (%)    44         42    efficiency    Cord strength  (N)    597        566    Cord efficiency                   (%)    84         82    ______________________________________

Using the mixing kneader allows for an easy increase in the processcapacity or an easy change thereof. The example further shows that themechanical properties of fibres made by the process according to theinvention are superior to those of fibres where the spinning solution isprepared using an extruder.

When the polymer solution as it leaves the mixing kneader and a polymersolution as it leaves an extruder are each spread between two sheets ofglass, the difference is clearly visible to the eye. The solutionobtained using the mixing kneader is more homogeneous, has been degassedmore thoroughly, and contains fewer solid particles.

We claim:
 1. A process for making fibres from a polyamide consistingessentially of poly (p-phenylene terephthalamide) in which a spinningmass is prepared by cooling concentrated sulphuric acid of at least 98wt. % content to below its coagulation point, subsequently combining thethus cooled sulphuric acid with the polyamide and mixing them to form asolid mixture containing, calculated on the weight of the mixture, atleast 15% of the polyamide having an inherent viscosity of at least 2.5,and then heating the resulting solid mixture to transform it to aliquid, and passing it to spinning orifices under pressure, and spinningit by means of an air gap spinning process, characterized in that theheating and pressuring of the solid mixture is carried out in a devicein which the polymer solution passes through at least two separatezones, a melting zone and a pressure built-up zone in succession, withthe polymer solution being kneaded as well as mixed in at least themelting zone.
 2. A process according to claim 1, characterised in thatthe polymer solution is degassed in the melting zone with the aid of avacuum.
 3. A process according to either claim 1, characterised in thatthe melting zone comprises a mixing kneader having a rotating shaftequipped with a number of kneading paddles or kneading blades, saidshaft being arranged in a cylindrical housing.
 4. A process according toclaim 3, characterised in that a discharge screw is fitted at the end ofthe rotating shaft.
 5. A process according to claim 4, characterised inthat at least one baffle is placed just upstream of the discharge screw.6. A process according to any one of the preceding claims, characterisedin that the spinning solution in the pressure build-up zone is passed tothe spinning orifices with the aid of a booster pump.